The present invention relates to a process for condensing linear polyalkylene terephthalates having a viscosity number of at least 60 cc/g, in two stages, at an elevated temperature, in the solid phase, and in the presence of alkanediols.
Polyalkylene terephthalates, especially polyethylene terephthalate (PETP) and polybutylene terephthalate (PBTP) are excellent materials for the production of films, sheets, molded parts, etc. Especially for the manufacture of molded parts by injection molding and extrusion processes, polyalkylene terephthalate must exhibit a viscosity number of &gt;100 cc/g.
Polyalkylene terephthalates are customarily produced by interesterification or esterification and subsequent polycondensation of terephthalic acid or its polyester-forming derivatives and an alkanediol, in the presence of catalysts (Sorensen and Campbell, Preparative Methods of Polymer Chemistry, Interscience Publishers Inc., NY [1961] : 111-127; "Kunststoff-Handbuch" [Plastics Manual] VIII, C. Hanser Publishers, Munich [1973], whose disclosures are incorporated by reference herein).
Using these processes, carried out in the melt, viscosities of a level required, for example, for injection molding and extrusion purposes, cannot be attained. Due to the temperature load, degradation reactions occur as early as the polycondensation stage, so that the desired high viscosity cannot be achieved.
A further limitation on the viscosity of the polyesters in the melt condensation method is due to the viscosity of the melt which increases with progressing polycondensation. This makes it difficult for the volatile, low-molecular weight reaction products to escape, or even makes such escape impossible, thereby inhibiting the progression of polycondensation. Additionally, technical difficulties are encountered in discharging the highly viscous melt from the reaction vessel (DOS [German Unexamined Laid-Open Application] No. 2,117,748).
Insofar as necessary, a solid-phase recondensation can be conducted (See, e.g., British Pat. No. 1,006,162 and U.S. Pat. No. 3,405,098, whose disclosures are incorporated by reference herein). This is done by bringing the polyester, in granulated or powdered form to a temperature lying approximately 10.degree.-60.degree. C. below the melting point. In this temperature range, the polyalkylene terephthalate is treated in an inert gas stream or under vacuum until the desired high viscosity has been attained. The recondensation can be conducted either discontinuously, for example in a tumbling dryer (DOS No. 2,117,748), or continuously, e.g. by the fluidizing method (DOS No. 1,804,551) or in a screw-type reactor (DOS No. 2,162,618).
A basic disadvantage of the solid phase recondensation resides in the long reaction times at elevated temperature; as a result, although a polyalkylene terephthalate is obtained having the desired viscosity, the product is already so damaged that it is strongly degraded upon processing from the melt. Molded parts made of such a material are brittle and fragile. It has therefore been suggested to use polyalkylene terephthalates in the ground form to shorten the recondensation time: but this method is not economical. In addition, there is the danger of caking of the powder (DOS's Nos. 2,152,245 and 2,117,748).
Another process includes reaction of the polyester, after the recondensation, with monofunctional glycidyl ethers or carbonates, thus improving the stability of the polyester (DOS No. 1,929,149). This process is time-consuming and requires the use of foreign-polyester substances.
DOS No. 2,503,000 proposes a method for the solid phase recondensation of PBTP wherein the recondensation is carried out in an inert gas stream constantly loaded with 1,4-butanediol. However, using this method it is impossible to attain the desired, high viscosity without the concomitant use of branching components. Polyalkylene terephthalates modified with branching components, as indicated in DOS No. 2,503,000, tend to gel and produce brittle and fragile molded parts.